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FEC on over North America
- Subject: [amsat-bb] FEC on over North America
- From: Phil Karn <karn@xxxxxxxx>
- Date: Sat, 05 Jul 2003 10:28:28 -0700
- User-Agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.4) Gecko/20030624
The FEC (forward error correction) mode is enabled on AO-40 this
morning. I'm sitting on my patio in San Diego with a laptop and R-7000
receiver connected to a G3RUH dish/feed and UEK-3000 downconverter
propped up on a patio chair.
The FEC telemetry is absolutely solid even though I'm getting a fair bit
of random impulse QRM (from nearby cordless phones?) I can offprint the
dish until the beacon is barely audible to the ear and it still works.
Here is a sample debug output from my program:
Carrier Clock symbol time Es/No
1560 399.610 1 18.0
1560 399.594 0 18.1
1570 399.610 1 18.8
1580 399.594 0 19.7
1590 399.577 0 21.2
1580 399.594 0 19.1
1590 399.577 0 20.3
1590 399.594 0 20.2
1580 399.577 0 18.8
1600 399.594 0 21.5
1580 399.577 0 18.2
1560 399.594 0 17.4
1570 399.594 0 18.2
1590 399.610 1 21.0
1610 399.594 0 22.8
1610 399.577 0 22.2
1610 399.594 0 21.7
1610 399.577 0 22.7
1600 399.594 0 21.6
1600 399.577 0 21.0
1610 399.561 -1 22.7
1600 399.577 0 20.9
1610 399.561 -1 22.2
1610 399.577 0 22.3
1610 399.594 0 21.3
1600 399.594 0 19.7
1580 399.610 1 18.0
1580 399.594 0 18.1
1580 399.610 1 18.3
1580 399.594 0 18.2
1590 399.577 0 18.9
correlator amp 35.5 dB energy = 87.2 dB scale = 206
VD histogram: 273 543 767 633 230 51 15 2 0 2 4 9 6 19 88 2493
RS byte corrections: 0 0 - Good Frame
channel symbol error corrections: 1 (0.0192%)
Decode attempts: 5987 Partial decodes: 0 Full decodes: 109
(end of excerpt)
It first shows, at one-second intervals, the estimated carrier
frequency, symbol clock frequency, symbol time offset and Es/No (the
signal-to-noise ratio per channel symbol). Since the telemetry format
consists of a conventional (uncoded) frame (which I am currently
ignoring) followed by a FEC frame, I have to process something like 31
seconds to get one 13-sec FEC frame.
The five lines at the bottom of the listing show that only one channel
symbol error had to be corrected. This was done by the Viterbi decoder;
the Reed-Solomon decoder had nothing to do. If this had been an uncoded
frame, that single error would have been enough to ruin it. But with
FEC, I can even retune the receiver in the middle of a frame and still
easily decode it.
The large "Decode attempts" figure does *not* imply that I've missed
that many frames. (In fact, I've missed none). Because the frames come
down asynchronously, I can't be sure where a frame is in time even after
successfully decoding a previous frame. So I take the very conservative
approach of attempting a decode whenever the sync correlator produces
even a small peak. If I'm wrong, all I do is waste a little CPU time.
But I won't miss any actual frames that way.
My software is up on my web page as
http://www.ka9q.net/ao40/ao40prototype-4.1.tar.gz
See also http://www.ka9q.net/ao40/ for more information on the FEC
format plus several other software packages needed to make this work.
This is *very early prototype* software, for Linux only, that I'm still
working on daily; the DBPSK demodulator in particular needs a lot of
work on its search and tracking algorithms. It is far from being a
turnkey package. That will hopefully come later. But the initial results
are very good.
73, Phil
----
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